1. Field of the Invention
The present invention relates to a method for synchronizing a telecommunication system, and to a telecommunication system and method for transmitting data from telecommunication system into at least one transmission network. In particular, the present invention relates to telecommunications systems which are used for transmitting data via a multiplicity of various transmission networks.
2. Description of the Prior Art
The term “telecommunication” is a generic term for all communication transmission methods via which various services in long-distance man-man, man-machine and machine-machine communication. Telecommunication is attaining a very special significance due to the growing-together of information technology and communication technology. Telecommunication is characterized by the transmission technology involving cable transmission technology, radio voice and data transmission, satellite technology, optical fiber technology, modems, digital switching systems and switching technology and local area networks.
To provide for a meaningful exchange of messages between two (or more) parties requires, in addition to the pure transmission of messages, a rule system which defines the conventions to be adhered to for meaningful communication in the form of protocols. Such rules are described, for example, in the service specifications of the individual layers of the OSI (open systems interconnection) Reference Model. The OSI Reference Model was created by the International Standardization Organization (ISO) in 1983 on the basis of the transmission of information in the field of data processing and is now also very widely used in applications of the communication systems. The OSI Model only represents principles of information transmission and, in consequence, only defines the logic of the information flow between subscribers. Since the OSI standard does not contain any specifications on the physical transmission of communication, it is manufacturer-independent but needs supplementary protocols for more detailed specification on the basis of other, e.g. proprietary standards, to implement a communication system.
In principle, a distinction can be made between asynchronous and synchronous communication. Asynchronous communication generally refers to the exchange of messages between a transmitting and a receiving entity which is completely decoupled in time. The time when a transmitting and its associated receiving operation is initiated cannot be predicted.
By comparison, synchronous communication refers to the exchange of messages between a transmitting and a receiving entity if this exchange occurs within a fixed timing pattern. In this context, a transmitting and its associated receiving operation always must be executed at the same time.
Telecommunication networks are characterized by the possibility of bidirectional and multidirectional data exchange between the subscribers. This presupposes that each subscriber involved can communicate with any other subscriber via the same medium. The simplest implementation of this is the communication of all subscribers in baseband. Due to the multiplicity of parallel active subscribers, methods are predominantly used which statically allocate the available bandwidth to the subscribers in time-division multiplex. Due to the increasing utilization of optical fiber technology, the necessity of improved intercontinental data communication and the increased performance requirements, the plesiochronous digital hierarchy (PDH), which has been prevailing since the 60s, is increasingly replaced by the synchronous digital hierarchy (SDH).
In the plesiochronous digital hierarchy, the data of various terminal systems present in PCM (pulse code modulation) are multiplexed bit by bit into a data stream. A hierarchical level is characterized by the number of basic channels multiplexed by it or, respectively, of the frames of the next lower hierarchical level.
Due to the difference in transmission rate of a basic channel in Europe and in the U.S., incompatible PDH hierarchies were formed. The DS1 standard prevailing in the U.S. provides for three hierarchical levels whereas five levels are provided in the European E1 standard. The lowest level (E1) multiplexes 30 basic channels. Due to the plesiochronous transmission in which deviations of (2-5)10−2 Hz from the nominal clock rate are permissible, the nominal transmission rates do not exactly correspond to a multiple of the next lower level but are slightly higher. The gap within a hierarchical level which thus occurs due to the permissible fluctuations is solved by stopping bits which do not contain any information (positive stopping).
Although optical fibers are also used as transmission medium for the higher PDH levels, the increase use of optical fibers led to considerations with regard to a new more powerful approach which was intended to replace the obsolete PDH technology even back in the 80s.
The development in the U.S. was initiated by the Bellcore company and taken over by the Industrial Carriers Compatibility Forum (ICCF) in 1984. The American SONET (Synchronous Optical Network) standard emerging from this resulted in the international or SDH (Synchronous Digital Hierarchy) standard issued by the International Telecommunications Union (ITU).
SONET is the standard issued by the CCITT for a family of interfaces for use in optical networks, the transmission medium of which is an optical fiber. SONET, thus, for the first time enables terminals from various manufacturers to be connected in standardized form and optical multiplexes with digital cross connects to be directly connected. In these arrangements, data rates of 51.84 Mb/s to over 2.4 Gb/s are achieved.
The synchronous digital hierarchy (SDH) is occasionally also called the international variant of SONET. The basic format of the SDH transmission is the STM-1 (synchronous transport module) frame. The STM-1 frame is composed of a header, with supplementary section information and a user data section, the container. Common to the STM frames also of all higher hierarchical levels is a fixed transmission time. Since the SDH standard is compatible with the SONET standard above a transmission rate of 155 Mb/s, SDH provides for simpler intercontinental data communication compared with PDH. Due to the lower overhead in comparison with the payload, SDH is also distinguished by very good efficiency of over 96%.
As a rule, telecommunication systems which are connected to standardized transmission networks such as PDH, SDH or SONET require synchronization in order to achieve the necessary clock accuracy at the interface to the transmission network. In this context, a distinction is made between two modes of synchronization. In the case of external synchronization, the system is supplied directly with a clock from an external synchronization source. In synchronization via the transmission link, by comparison, the clock is obtained from the received data stream of the interface and supplied as synchronization source to the system. For this purpose, the received data frames also contain, among other things, supplementary information describing the quality of the clock signal of a distant station in addition to the user information.
In some of the interface types in plesiochronous digital hierarchy, the clock signal quality is transmitted in the timing marker bit. Table 1 shows the timing marker bit MA byte according to PDH (ITU G.832, E3).
In the case of SONET and the synchronous digital hierarchy SDH, the quality of the clock signal is communicated in the so-called SSM (synchronization status message) byte. Table 2 shows the SSM definition in SONET (Bellcore GR253) and Table 3 shows the SSM definition in the synchronous digital hierarchy (ITU G.708).
Since different standardized transmission networks such as PDH, SDH or SONET exist in parallel, there is a need for telecommunication systems which maintain connections to a number of these transmission networks at the same time. FIG. 2 shows such a telecommunication system which includes a main processor MP 2 and interface cards 4-10. Such a telecommunication system has a separate interface card for each different interface type. The telecommunication system also types a synchronization system 12 which is connected both to the main processor 2 and the interface cards.
As shown in FIG. 2 under reference symbol 14, the interface cards 4-10 communicate the respective clock signal qualities to the main processor 2. In addition, the interface cards, as shown under reference symbol 16, supply the clock obtained from the transmission network in each case to the synchronization system 12. The synchronization system synchronizes the telecommunication system with a clock 16 transmitted from the interface cards under control by the main processor 2 via connections 17.
On the basis of the clock signal qualities received from the interface cards 4-10 via the connections 14, the main processor 2 assesses with which one of the clock signals supplied from the standardized transmission networks the telecommunication system is to be synchronized. Since, as stated above and as can be seen from tables 1 to 3, the clock signal quality information items provided by the different interface types are present in different formats and even have different value ranges, the clock signal qualities from different interface types must be treated separately in the main processor 2. For this purpose, the main processor has subunits 20, each one of which is provided for processing the clock signal quality information items of one interface type (PDH, SDH, SONET). These different subunits 20 of the main processor 2 in each case have a different structure and in each case have different functionalities due to the different data formats. As such, the clock signal qualities are treated separately in the main processor. Different treatment is also evident from the current draft of ITU-T, G.synce, 01/98 (synchronization layer functions). This draft distinguishes between options 1 to 3, options 1 and 3 describing the synchronous digital hierarchy according to the previous ITU standard whereas option 2 relates to the SONET based on Bellcore.
Telecommunication systems of the aforementioned design have a disadvantage that a number of algorithms for the value ranges and formats inherent in the different transmission networks are necessary in the main processor. Owing to the different standards in existence, which also can differ in various countries, an adaptation of the main processor is therefore always necessary.
The present invention is, thus, directed to a telecommunication system of the abovementioned type, a method for synchronizing such a telecommunication system, and a method for transmitting data from such a telecommunication system, in which the communication of clock signal qualities is simplified.